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                    <h1 class="description center-align post-title">⚫ React Hooks 精进 7 - 全局状态管理 Redux</h1>
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                <p><strong>Redux 是状态管理框架，基于不可变数据的机制。</strong></p>
<h1 id="Redux-的出现背景"><a href="#Redux-的出现背景" class="headerlink" title="Redux 的出现背景"></a>Redux 的出现背景</h1><p>原先的数据传递：组件级别的 state，从上而下的 props 两个状态机制。</p>
<p>但是无法满足类似跨层级的组件数据共享和传递。</p>
<h2 id="Redux-Store-的特点"><a href="#Redux-Store-的特点" class="headerlink" title="Redux Store 的特点"></a>Redux Store 的特点</h2><ul>
<li><strong>Redux Store 是全局唯一的。</strong></li>
<li><strong>Redux Store 是树状结构。</strong></li>
</ul>
<h2 id="使用场景"><a href="#使用场景" class="headerlink" title="使用场景"></a>使用场景</h2><ul>
<li>跨组件的状态共享</li>
<li>同组件多个实例的状态共享</li>
</ul>
<h1 id="基本概念"><a href="#基本概念" class="headerlink" title="基本概念"></a>基本概念</h1><ul>
<li>State：即 Store</li>
<li>Action：用于描述发生的动作</li>
<li>Reducer：接受 Action 和 State 作为参数通过计算得到新的 Store</li>
</ul>
<h2 id="好处："><a href="#好处：" class="headerlink" title="好处："></a>好处：</h2><ul>
<li>保证数据的不可变性：<strong>在 Reducer 内每次必须返回一个新的对象</strong></li>
<li>可预测性</li>
<li>易于调试：可以跟踪 Store 中数据的变化</li>
</ul>
<h2 id="Redux-逻辑步骤"><a href="#Redux-逻辑步骤" class="headerlink" title="Redux 逻辑步骤"></a>Redux 逻辑步骤</h2><ul>
<li>创建 store</li>
<li>利用 Action 和 Reducer 修改 Store</li>
<li>利用 subscribe 监听 store 变化</li>
</ul>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">import</span> <span class="token punctuation">&#123;</span> createStore <span class="token punctuation">&#125;</span> <span class="token keyword">from</span> <span class="token string">"redux"</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> initStore <span class="token operator">=</span> <span class="token punctuation">&#123;</span> value<span class="token operator">:</span> <span class="token number">0</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">counterReducer</span><span class="token punctuation">(</span>state <span class="token operator">=</span> initStore<span class="token punctuation">,</span> action<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
  <span class="token keyword">switch</span> <span class="token punctuation">(</span>action<span class="token punctuation">.</span>type<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">case</span> <span class="token string">"counter/incremented"</span><span class="token operator">:</span>
      <span class="token keyword">return</span> <span class="token punctuation">&#123;</span> value<span class="token operator">:</span> state<span class="token punctuation">.</span>value <span class="token operator">+</span> <span class="token number">1</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
    <span class="token keyword">case</span> <span class="token string">"counter/decremented"</span><span class="token operator">:</span>
      <span class="token keyword">return</span> <span class="token punctuation">&#123;</span> value<span class="token operator">:</span> state<span class="token punctuation">.</span>value <span class="token operator">-</span> <span class="token number">1</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
    <span class="token keyword">default</span><span class="token operator">:</span>
      <span class="token keyword">return</span> state<span class="token punctuation">;</span>
  <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span>

<span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token function">createStore</span><span class="token punctuation">(</span>counterReducer<span class="token punctuation">)</span><span class="token punctuation">;</span>
store<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token builtin">console</span><span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>store<span class="token punctuation">.</span><span class="token function">getState</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> incrementAction <span class="token operator">=</span> <span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"counter/incremented"</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> decrementAction <span class="token operator">=</span> <span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"counter/decremented"</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>

store<span class="token punctuation">.</span><span class="token function">dispatch</span><span class="token punctuation">(</span>incrementAction<span class="token punctuation">)</span><span class="token punctuation">;</span>
store<span class="token punctuation">.</span><span class="token function">dispatch</span><span class="token punctuation">(</span>incrementAction<span class="token punctuation">)</span><span class="token punctuation">;</span>
store<span class="token punctuation">.</span><span class="token function">dispatch</span><span class="token punctuation">(</span>decrementAction<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h1 id="使用方法"><a href="#使用方法" class="headerlink" title="使用方法"></a>使用方法</h1><ul>
<li>React 组件能在依赖的 Store 的数据发生变化时重新 render</li>
<li>在 React 组件中，能够在某些时机去 dispatch 一个 action，从而出发 store 的更新</li>
</ul>
<p><strong>需要在 React 内引入 react-redux 工具库，让 React 和 Redux 实现互通。</strong></p>
<p>为了确保需要绑定的组件能够访问到全局唯一的 Redux Store，需要利用 React 的 Context 机制去存放 Store 信息。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">import</span> <span class="token punctuation">&#123;</span> Provider <span class="token punctuation">&#125;</span> <span class="token keyword">from</span> <span class="token string">"react-redux"</span><span class="token punctuation">;</span>

ReactDOM<span class="token punctuation">.</span><span class="token function">render</span><span class="token punctuation">(</span>
  <span class="token operator">&lt;</span>Provider store<span class="token operator">=</span><span class="token punctuation">&#123;</span>store<span class="token punctuation">&#125;</span><span class="token operator">></span>
    <span class="token operator">&lt;</span>App <span class="token operator">/</span><span class="token operator">></span>
  <span class="token operator">&lt;</span><span class="token operator">/</span>Provider<span class="token operator">></span><span class="token punctuation">,</span>
  rootElement
<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p><strong>然后就可以利用 react-redux 提供的**</strong><code>useSelector</code>*<strong>*和**</strong><code>useDispatch</code>*<strong>*两个 Hooks 来使用 Redux 了。</strong></p>
<ul>
<li><code>useSelector</code>：让组件在 Store 的某些数据发生变化时重新 render</li>
</ul>
<pre class="line-numbers language-mermaid" data-language="mermaid"><code class="language-mermaid"><span class="token keyword">graph</span> TD
  Reducer <span class="token arrow operator">--></span> Store
	Store <span class="token arrow operator">--></span> View
	View <span class="token arrow operator">--></span> Action
	Action <span class="token arrow operator">--></span> Reducer<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h1 id="利用-Redux-处理异步逻辑（异步-Action）"><a href="#利用-Redux-处理异步逻辑（异步-Action）" class="headerlink" title="利用 Redux 处理异步逻辑（异步 Action）"></a>利用 Redux 处理异步逻辑（异步 Action）</h1><p>在 Store 内我们不仅维护着业务数据同时维护着应用程序的状态。</p>
<p>store 的主要变化：</p>
<ul>
<li>请求发送时：设置<code>state.pending = true</code> → 用于 UI 显示加载中状态</li>
<li>请求发送成功：设置<code>state.pending = false, state.data = result</code> → 取消加载状态同时将获取的数据展示在 UI 上</li>
<li>请求发送失败：设置<code>state.pending = false, state.error = error</code> → 取消加载状态，设置错误状态和信息，用于在 UI 上显示错误内容</li>
</ul>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">DataList</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
  <span class="token keyword">const</span> dispatch <span class="token operator">=</span> <span class="token function">useDispatch</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">useEffect</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
    <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_START"</span> <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">fetch</span><span class="token punctuation">(</span><span class="token string">"/xxx"</span><span class="token punctuation">)</span>
      <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
        <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_SUCCESS"</span><span class="token punctuation">,</span> data<span class="token operator">:</span> res <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">&#125;</span><span class="token punctuation">)</span>
      <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
        <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_ERROR"</span><span class="token punctuation">,</span> error<span class="token operator">:</span> err <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">&#125;</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> data <span class="token operator">=</span> <span class="token function">useSelector</span><span class="token punctuation">(</span><span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=></span> state<span class="token punctuation">.</span>data<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> pending <span class="token operator">=</span> <span class="token function">useSelector</span><span class="token punctuation">(</span><span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=></span> pending<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">const</span> error <span class="token operator">=</span> <span class="token function">useSelector</span><span class="token punctuation">(</span><span class="token punctuation">(</span>state<span class="token punctuation">)</span> <span class="token operator">=></span> error<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>error<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token string">"Error"</span><span class="token punctuation">;</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>pending<span class="token punctuation">)</span> <span class="token keyword">return</span> <span class="token string">"Loading..."</span><span class="token punctuation">;</span>
  <span class="token keyword">return</span> <span class="token operator">&lt;</span>Table data<span class="token operator">=</span><span class="token punctuation">&#123;</span>data<span class="token punctuation">&#125;</span> <span class="token operator">/</span><span class="token operator">></span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>在上述代码中使用三个同步 Action 完成这个异步请求的场景，这里将 store 完全作为一个存放数据的地方。</p>
<p>发送请求获取数据并进行错误处理的逻辑是不可重用的，如果在另一个组件中也使用同样的请求就需要在组件内重新实现一遍。</p>
<p>因此 Redux 提供了<strong>middleware</strong>机制可以巧妙地实现异步 Action 的概念。</p>
<p>middleware 可以让你提供一个拦截器在 reducer 处理 action 之前被调用，在拦截器内可以自由地处理获得的 action。</p>
<p><strong>Action → Middleware → Reducer</strong></p>
<p>然后 Redux 提供了 react-thunk 的中间件，它如果发现接收到的 action 是一个函数，那么就不会传递给 reducer 而是执行这个函数，并把 dispatch 作为参数传给这个函数。</p>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">import</span> <span class="token punctuation">&#123;</span> createStore<span class="token punctuation">,</span> applyMiddleware <span class="token punctuation">&#125;</span> <span class="token keyword">from</span> <span class="token string">"react"</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> thunkMiddleware <span class="token keyword">from</span> <span class="token string">"redux-thunk"</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> rootReducer <span class="token keyword">from</span> <span class="token string">"./reducer"</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> composedEnhancer <span class="token operator">=</span> <span class="token function">applyMiddleware</span><span class="token punctuation">(</span>thunkMiddleware<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">const</span> store <span class="token operator">=</span> <span class="token function">createStore</span><span class="token punctuation">(</span>rootReducer<span class="token punctuation">,</span> composeEnhancer<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<pre class="line-numbers language-typescript" data-language="typescript"><code class="language-typescript"><span class="token keyword">function</span> <span class="token function">fetchData</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
  <span class="token keyword">return</span> <span class="token punctuation">(</span>dispatch<span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
    <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_START"</span> <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token function">fetch</span><span class="token punctuation">(</span><span class="token string">"/xxx"</span><span class="token punctuation">)</span>
      <span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token punctuation">(</span>res<span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
        <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_SUCCESS"</span><span class="token punctuation">,</span> data<span class="token operator">:</span> res <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">&#125;</span><span class="token punctuation">)</span>
      <span class="token punctuation">.</span><span class="token function">catch</span><span class="token punctuation">(</span><span class="token punctuation">(</span>err<span class="token punctuation">)</span> <span class="token operator">=></span> <span class="token punctuation">&#123;</span>
        <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token punctuation">&#123;</span> type<span class="token operator">:</span> <span class="token string">"FETCH_ERROR"</span><span class="token punctuation">,</span> error<span class="token operator">:</span> err <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<pre class="line-numbers language-javascript" data-language="javascript"><code class="language-javascript"><span class="token keyword">function</span> <span class="token function">DataList2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
  <span class="token keyword">const</span> dispatch <span class="token operator">=</span> <span class="token function">useDispatch</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">dispatch</span><span class="token punctuation">(</span><span class="token function">fetchData</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>
<p><strong>通过这种方式就可以实现异步请求的重用，这一套结合 redux-thunk 中间件的机制称之为异步 Action（更像是 Redux 的使用模式，通过组合使用同步 Action 用一致的方式提供处理异步逻辑的方案）。</strong></p>
<h1 id="思考题"><a href="#思考题" class="headerlink" title="思考题"></a>思考题</h1><p>只考虑 Redux，如何在计数器内实现指定的变量值。</p>

                
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            <span class="title"><i class="fas fa-search"></i>&nbsp;&nbsp;搜索</span>
            <input type="search" id="searchInput" name="s" placeholder="请输入搜索的关键字"
                   class="search-input">
        </div>
        <div id="searchResult"></div>
    </div>
</div>

<script type="text/javascript">
$(function () {
    var searchFunc = function (path, search_id, content_id) {
        'use strict';
        $.ajax({
            url: path,
            dataType: "xml",
            success: function (xmlResponse) {
                // get the contents from search data
                var datas = $("entry", xmlResponse).map(function () {
                    return {
                        title: $("title", this).text(),
                        content: $("content", this).text(),
                        url: $("url", this).text()
                    };
                }).get();
                var $input = document.getElementById(search_id);
                var $resultContent = document.getElementById(content_id);
                $input.addEventListener('input', function () {
                    var str = '<ul class=\"search-result-list\">';
                    var keywords = this.value.trim().toLowerCase().split(/[\s\-]+/);
                    $resultContent.innerHTML = "";
                    if (this.value.trim().length <= 0) {
                        return;
                    }
                    // perform local searching
                    datas.forEach(function (data) {
                        var isMatch = true;
                        var data_title = data.title.trim().toLowerCase();
                        var data_content = data.content.trim().replace(/<[^>]+>/g, "").toLowerCase();
                        var data_url = data.url;
                        data_url = data_url.indexOf('/') === 0 ? data.url : '/' + data_url;
                        var index_title = -1;
                        var index_content = -1;
                        var first_occur = -1;
                        // only match artiles with not empty titles and contents
                        if (data_title !== '' && data_content !== '') {
                            keywords.forEach(function (keyword, i) {
                                index_title = data_title.indexOf(keyword);
                                index_content = data_content.indexOf(keyword);
                                if (index_title < 0 && index_content < 0) {
                                    isMatch = false;
                                } else {
                                    if (index_content < 0) {
                                        index_content = 0;
                                    }
                                    if (i === 0) {
                                        first_occur = index_content;
                                    }
                                }
                            });
                        }
                        // show search results
                        if (isMatch) {
                            str += "<li><a href='" + data_url + "' class='search-result-title'>" + data_title + "</a>";
                            var content = data.content.trim().replace(/<[^>]+>/g, "");
                            if (first_occur >= 0) {
                                // cut out 100 characters
                                var start = first_occur - 20;
                                var end = first_occur + 80;
                                if (start < 0) {
                                    start = 0;
                                }
                                if (start === 0) {
                                    end = 100;
                                }
                                if (end > content.length) {
                                    end = content.length;
                                }
                                var match_content = content.substr(start, end);
                                // highlight all keywords
                                keywords.forEach(function (keyword) {
                                    var regS = new RegExp(keyword, "gi");
                                    match_content = match_content.replace(regS, "<em class=\"search-keyword\">" + keyword + "</em>");
                                });

                                str += "<p class=\"search-result\">" + match_content + "...</p>"
                            }
                            str += "</li>";
                        }
                    });
                    str += "</ul>";
                    $resultContent.innerHTML = str;
                });
            }
        });
    };

    searchFunc('/search.xml', 'searchInput', 'searchResult');
});
</script>

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